Hohmann orbit, most economical path (though not the shortest or fastest) for a spacecraft to take from one planet to another. The German engineer Walter Hohmann showed in 1925 that elliptical orbits tangent to the orbits of both the planet of departure and the target planet require the least fuel and energy.
The Hohmann orbit from Earth to Mars requires about 260 (Earth) days; about two years and eight months would be needed for a round trip, allowing for a waiting period of 455 days on Mars while the planets realigned themselves properly so that the returning craft would meet Earth’s orbit when the Earth was present. To reach Venus 146 days would be required, and two years and one month for a round trip with waiting time included. Compromise orbits, faster but needing more energy than true Hohmann orbits, have been used by interplanetary probes launched since the 1960s.